Field of the Invention
The present invention relates to an image reading apparatus and an image forming system.
Description of the Related Art
Heretofore, an image reading apparatus is known which reads a sheet output from an image forming apparatus and processes the read image data for the purpose of improving the print performance of the image forming apparatus. The image reading apparatus is provided with a line sensor (image reading unit) consisting of elements which are linearly arranged in the sheet width direction perpendicular to the sheet conveying direction, and paper conveying units located in the upstream and downstream sides of this line sensor respectively. When the paper conveying unit conveys a sheet on which a predetermined pattern is formed, this image reading apparatus outputs image data by reading the sheet with the line sensor. Necessary data can be calculated based on this image data for performing calibration of the gradation characteristics of images and image position adjustment of the front and back sides.
For example, Japanese Patent Published Application No. 2008-104165 discloses an image reading apparatus provided with image reading units which read the front and back sides of a sheet (original) respectively in order that the image reading unit for reading the front side is located in the upstream side of the image reading unit for reading the back side. In this type of the image reading apparatus, the conveying speed of a sheet increases when the sheet enters a discharge roller in the downstream side of in the conveying direction. Because of this, the read image is influenced by magnification variation (variation in reading magnification of an image) within the surface of a sheet in the conveying direction of the sheet. As a result, the read size of the read image in the front side and the read size of the read image in the back side are reduced in the conveying direction as compared with the actual sheet size. Particularly, the read size of the read image of the back side tends to be further reduced as compared with that of the front side.
For this reason, in the case of the image reading apparatus disclosed in Japanese Patent Published Application No. 2008-104165, the read size of the read image in the front side is adjusted in order to coincide with the actual sheet size by decreasing the conveying speed of a conveyance roller which determines the conveying speed of the sheet before reaching a discharge roller. Also, with respect to the read size of the read image in the back side, the reading time of the image reading unit for each line in the back side is shortened in order that the read size of the read image of the back side coincides with the actual sheet size. The reading time is determined in accordance with the sheet size and uniformly applicable to the entirety of the paper surface. The Published Application also describes an alternative method of switching the reading time for each line to a shorter time with the timing when a sheet enters the discharge roller, and uniformly applying the shorter time thereafter for reading an image. The shorter time after switching is determined in accordance with the sheet size in the same manner.
However, in the case of the method disclosed in Japanese Patent Published Application No. 2008-104165, since the reading time, even if shortened, is uniformly fixed after switching, there is the possibility that an image is disturbed. Also, if the reading time is shortened, the quantity of light entering the line image sensor decreases so that density unevenness of an image may occur.
Taking into consideration the above circumstances, it is an object of the present invention therefore to provide an image reading apparatus and an image forming system in which image disturbance and density unevenness can be inhibited.